Semiconductor gas sensors having a combination of p- and n-type oxides have been investigated for detecting a small amount of NO. Among 16 kinds of p-n combinations tested, the Co₃O₄-Fe₂O₃ composite sensor exhibited excellent sensing properties to NO at 300°C. When the Co₃O₄ content in the composite was set to 20wt%, the sensitivity to 50 ppm NO was the maximum as high as 2.8, while that to 50 ppm NO₂ was 1.5, indicating the highly sensitive and selective nature to NO. Further, the sensor showed the almost linear correlation between logarithm of NO sensitivity and that of NO concentration in the range of 5-100 ppm NO. These results suggest that the Co₃O₄-Fe₂O₃ composite sensor can be a candidate of NO sensor for combustion monitoring. The addition of Co₃O₄ promoted the formation of NO^- adsorbate on the composite surface, resulting in the high NO sensitivity.